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Abstract

Holographic optical tweezers (HOTs) techniques are further developed to study hyaluronan-mediated adhesion of chondrocyte cells. We present a calibration scheme and address fundamental issues concerning the use of HOTs for quantitative force measurements. Influence of SLM pixelation on trap stiffness is observed and can be utilized to design calibrated HOTs more effectively. It is also shown that the HOTs trapping stiffness can vary significantly over short distances. Then we use HOTs cell adhesion assays investigate the viscoelastic and adhesive nature of chondrocytes' pericellular matrix (PCM) at two different time steps (30 minute and 24 hour incubation periods). Surprisingly, no physical influence of the large, presumably gel-like PCM is observed. However, a difference is discerned in the adhesiveness of the two sets of cells. The early-stage cells have reversible adhesion with negatively-charged and fibronectin-coated microspheres even after they are held at the cell surface for 10 seconds. In contrast, late stage cells stick irreversibly to all types of beads: positive, negative, fibronectin and hyaluronan-coated. Additionally, only the late stage cells produce membrane tethers. These observations suggest that the late-stage chondrocytes have less surface-associated hyaluronan and have interesting implications for the role of hyaluronan in the early stages of cell adhesion.